Substantially two methods are used determining and compensating for background absorption. With one method, a reference light beam originating from a light source emitting a continuum with a relatively large bandwidth as compared to the line width is directed through the flame or cloud of atoms alternately with the measuring light beam, the absorption due to the atomic absorption plus background absorption, while the absorption of the reference light beam virtually is determined only by the background absorption (DE-A-1,911,048).
Atomic absorption spectrometers are known wherein a measuring light beam originates from a line emitting light source and is directed through a flame or a furnace for the electrothermal atomization to a detector and wherein a reference light beam originating from a light source emitting a continuum becomes effective alternately with this measuring light beam containing a line spectrum of an element looked for. This reference light beam is reflected into the path of rays of the measuring light beam by means of a beam splitter. The beam splitter usually is a partially transparent mirror. This mirror has reflecting and transparent areas uniformly distributed, such that 50% of the measuring light beam pass through the transparent surface portions and 50% of the reference light beam are reflected in the direction of the measuring light beam by the reflecting surface portions. The alternation between measuring light beam and reference light beam is achieved in that both light sources are switched on alternately. ("Applied Spectroscopy" Vol. 27, No. 6 (1973), 467-470).
In this prior art arrangement each light beam is attenuated by 50%. This results in deterioration of the signal-to-noise-ratio which may be critical with very sensitive measurements.
Furthermore it is known to leave a glass plate arranged in the bisecting line of the angle of the beam axes of hollow cathode lamp and deuterium lamp transparent on one surface portion and to provide it with a partially reflecting layer on another surface portion. The glass plate can be displaced in its plane, whereby optionally either both light beams fall on the partially reflecting layer and their paths of rays are superimposed, or only the light beam from the hollow cathode lamp passes through the transparent portion of the glass plate and falls onto a photometer. Then the instrument can optionally be used as a single beam photometer (JP-A-55 419; "Patent Abstracts of Japan" Vol. 4, No 100 (P-19), (582), 18.07. 1980).
Another method for determining the background absorption is based on Zeeman's effect: By applying a magnetic field to the sample the absorption line of the looked-for element in the sample is shifted relative to the spectral lines of the measuring light beam, such that there is no atomic absorption with the magnetic field applied and only background absorption is measured. The atomic absorption corrected for the background absorption can be measured by switching the magnetic field on or off.
The invention relates to an atomic absorption spectrometer in which the background absorption is determined by means of a reference light beam from light source emitting a continuum.
From German Patent Application 1,964,469 an atomic absorption spectrometer is known in which the radiation originates from a single light source designed as a line emitter, the radiation of which passing through the sample is frequency modulated by use of the longitudinal Zeeman's effect. In this prior art atomic absorption spectrometer, a hollow cathode lamp is arranged between the pole pieces of a solenoid. One of the pole pieces has a bore through which the measuring light beam passes through a flame serving as atomizing device and a monochromator and impinges upon a photo-electrical detector. The solenoid is arranged to be switched on and off, whereby the atomic absorption of the sample atoms compensated for the background absorption can be determined from the difference of the signals with the solenoid switched off and switched on. The windings of the solenoid are provided on the pole pieces.
In this prior art atomic absorption spectrometer the emission lines of the line emitting light source are periodically shifted by Zeeman's effect and thus the emitted light frequency is modulated and not the absorption lines of the sample.
From German patent 2,165,106 it is known to apply the magnetic field of a solenoid arranged to be switched on and off to the atomizing device, i.e. to the sample which is to be atomized, instead of to the light source. Therein the atomizing device is a flame. The magnetic field is applied perpendicularly to the direction of propagation of the measuring light beam. A splitting of the absorption lines due to "transverse" Zeeman's effect is effected, which again causes a relative shift of the emission lines of the measuring light beam and of the absorption lines of the sample. Again it can be discriminated between atomic absorption by the atoms of the element looked for and non-specific background absorption by switching the magnetic field on and off.
When the transverse Zeeman's effect is used splitting of a spectral line is effected into a central line the wave length of which corresponds to the non-shifted wave length of the respective line with the magnetic field switched off and two side lines are shifted to longer and shorter wave length relative thereto. The central line and the side lines are polarized differently. Therefore the influence of the central line can be eliminated by a polarizer. Such a polarizer, however, causes a light loss of 50%.
GB-A-2,096,315 describes an atomic absorption spectrometer which uses a graphite tube for the electrothermal atomization of the sample. For background compensation, a solenoid energized by a.c. current is provided, which generates a magnetic field orthogonal to the beam axis of the measuring light beam. By the transverse Zeeman's effect the absorption lines are split into a central .pi.-component which is located at the wavelength of the undisturbed absorption line, and .sigma.-components shifted relative thereto. The .pi.-and .sigma.-components are differently polarized. A monochromator is adjusted to the undisturbed absorption line. There is no polarizer for the elimination of the .sigma.-component.
The compensation of the background absorption by use of Zeeman's effect can present problems, if the background absorption varies quickly, as the frequency of the switching on and switching off of the magnetic field is limited due to the inductivity of the solenoid (International Laboratory" 17 (1987) No. 3).
The various possibilities of the Zeeman-AAS have been shown in a paper in "TRAC Trends in Analytical Chemistry" Vol. 1 (1982), May, No. 9, 203-205.